Wet scrubbing methods for the removal of sulfur dioxide from gases, such as flue gases from the combustion of fossile fuels, using lime or magnesium as the scrubbing component, have concentrated on two major systems. One method uses a magnesium-enhanced lime aqueous scrubbing slurry, while the other uses an aqueous solution of magnesium scrubbing components, with regeneration of the spent solution using lime or a magnesium-enhanced lime.
Illustrative of the methods that use an aqueous scrubbing slurry containing magnesium-enhanced lime are those methods described in U.S. Pat. Nos. 3,914,378; 3,919,393; 3,919,394; 4,976,936; 4,996,032; and 5,645,807. In such methods, an aqueous slurry of lime contains magnesium ions or magnesium hydroxide in an amount of 250-5000 parts per million (ppm), or 2-10% by weight, in the wet scrubbing unit used for contact of the gas and aqueous scrubbing slurry. The aqueous magnesium-enhanced lime scrubbing slurry is contacted with the sulfur-dioxide containing gas in a wet scrubbing unit with formation of calcium sulfite that is either disposed of or oxidized and converted to gypsum, a useful by-product.
Illustrative of the methods that use an aqueous solution of magnesium scrubbing components, such as magnesium hydroxide, magnesium bisulfite or magnesium sulfite, are those described in U.S. Pat. Nos. 4,804,523, 5,039,499; 5,084,255; 5,270,026; and 5,614,158.
While both the method using an aqueous slurry of magnesium-enhanced lime and the method using an aqueous solution of magnesium scrubbing compounds have been shown to be commercially viable, improvements to those methods are constantly sought.
In the methods according to U.S. Pat. Nos. 5,039,499 and 5,084,255, for example, where byproduct gypsum and magnesium hydroxide are to be produced, the method uses a magnesium hydroxide slurry alone as the scrubbing component for sulfur dioxide removal in the wet scrubber and is very efficient. However, at times, the rate of bleed stream removal from the wet scrubbing unit is not sufficiently great enough and an objectionable level of magnesium sulfite can exist in the wet scrubbing unit.
The present method of removing sulfur dioxide from a gaseous stream in a wet scrubbing unit where an aqueous scrubbing slurry of magnesium and calcium scrubbing components are provided to convert the sulfur dioxide to sulfites and sulfates and producing calcium sulfate dihydrate from an aqueous effluent from the wet scrubbing unit, which aqueous effluent contains the sulfites and sulfates includes providing an aqueous scrubbing slurry by blending lime and magnesium hydroxide in a ratio to provide a molar ratio of Ca:Mg of between about 1.0 to 2.0, so as to provide a solids content in said aqueous scrubbing slurry in the wet scrubbing unit of between about 12 to 20% preferably about 15%, which solids content includes about 10 to 15% preferably about 11%, suspended solids and about 2 to 5%, preferably about 4%, dissolved solids, a dissolved sulfite content of about 13,000 to 20,000, ppm, and a pH of between about 5.8-6.0.
An aqueous effluent is discharged from the wet scrubbing unit containing the solids content, dissolved sulfites and pH and is passed to an oxidizing unit.
The aqueous effluent is contacted in the oxidizing unit with an oxidizing gas to convert sulfites therein to sulfates and produce an oxidized aqueous effluent containing dissolved magnesium sulfate, precipitated calcium sulfate dihydrate, and residual calcium sulfate dihydrate. The calcium sulfate dihydrate is precipitated from the aqueous effluent, and the oxidized aqueous containing dissolved magnesium sulfate and residual calcium sulfate dihydrate is passed to a regeneration tank, where the oxidized aqueous effluent is contacted with a lime slurry, to produce magnesium hydroxide and further precipitated calcium sulfate dihydrate in an aqueous medium. The further precipitated calcium sulfate dihydrate is separated from the aqueous medium to provide an aqueous medium containing magnesium hydroxide, and at least a portion of the aqueous medium containing magnesium hydroxide is returned to the wet scrubbing unit.